(1) Field of the Invention
The present invention relates to a method for purifying diamond, and more particularly relates to a method for purifying diamond, wherein only graphite is oxidized and removed from a mixture of diamond and graphite at a relatively low temperature.
(2) Description of the Prior Art
Diamond has hitherto been artificially synthesized through a static pressure method by means of an ultra-high pressure press, a shock wave method which utilizes an instantaneous ultra-high pressure by an explosive, and the like.
However, in any of these methods, the starting graphite is not completely converted into diamond, but a mixture of diamond and graphite is always obtained.
In general, in the synthesis of diamond, in addition to the starting graphite, metals, such as iron, cobalt, nickel and the like, are used in the static pressure method; and metals, such as iron, copper and the like, are used even in the shock wave method.
These metals can be relatively easily removed by means of a mineral acid, however, it is not easy to separate diamond from graphite in the residual mixture.
Powdery mixture of graphite and diamond has various physical or chemical properties depending upon the synthetic methods for diamond. For example, diamond synthesized through the static pressure method consists of particles having a size larger than that of diamond synthesized through the shock wave method and consisting of single crystal or substantially consisting of single crystal. On the contrary, diamond synthesized through the shock wave method consists of particles generally having a size of not larger than 20-30 .mu.m and consisting of polycrystals having a complicated shape in their surface. Accordingly, the diamond synthesized through the shock wave method has a high chemical reactivity.
The mixture of diamond and graphite obtained in the synthesis of diamond includes not only a mixture wherein diamond particles are merely mixed with graphite particles, but also a mixture wherein diamond particles are agglomerated together with graphite particles into one coarse particle, and a mixture wherein a grahite layer is put between diamond layers. Moreover, in the mixture, diamond particles and graphite particles have various and different sizes and further have different apparent densities.
Due to the above described reason, it is necessary to limit properly the method and condition for separating diamond from graphite. In the separation, physical method and chemical method are generally used.
In the physical method, diamond is separated from graphite, for example, by utilizing the difference in their specific gravity. However, this method is poor in accuracy and efficiency, and can not be practically used. Therefore, the chemical method is predominantly used at present for the separation.
In the chemical method, a powdery mixture of diamond and graphite is generally heated together with an oxidizer, whereby the graphite is oxidized and removed.
In this chemical method, there have been known wet process and dry process. In the wet process, there is used a combination of potassium dichromate and strong phosphoric acid; a combination of silver dichromate and concentrated sulfuric acid; a combination of potassium iodate and strong phosphoric acid; or a combination of nitric acid, a chlorate and an aqueous hydrogen peroxide solution. In the dry process, there is used a process, wherein a powdery mixture of diamond and graphite is heat treated together with lead oxide, alkali metal carbonate or the like.
These processes have both merits and demerits. For example, when strong phosphoric acid is used, the operation is difficult due to the high viscosity of the treating mixture; when concentrated sulfuric acid is used, the treating mixture must be kept to a high temperature, and hence the handling of the mixture is dangerous; and when potassium iodate is used, a large amount of poisonous free iodine is generated during the treatment, and hence the operation environment is injured.
Japanese Patent Application Publication No. 23,965/69 discloses a method, wherein diamond is produced from graphite by a shock wave method, and then the reaction mixture is subjected to an oxidation treatment at a temperature of about 280.degree. C. in nitric acid. However, such high temperature treatment has a drawback that fine diamond particles are oxidized.
Japanese Patent Application Publication No. 10,949/79 discloses a method, wherein a powdery mixture of diamond and graphite is heated together with nitric acid, sodium chlorate and an aqueous hydrogen peroxide solution to carry out an oxidation treatment of the powdery mixture. However, this method has a drawbacks that environmental pollution occurs due to the development of a large amount of fume, and that a dangerous substance, such as aqueous hydrogen peroxide solution, must be handled.
In the dry process wherein a powdery mixture of diamond and graphite is oxidized in air, when lead oxide is used as an oxidation assistant, the oxidation treatment must be carried out at a high temperature of 350.degree.-550.degree. C. for a long time of several tens of hours, and moreover a system for recovering completely poisonous lead compounds is required, and other various troubles are caused.
Further, in a dry process wherein alkali metal carbonate is used, the treatment must be carried out at a high temperature of 550.degree.-600.degree. C. under an oxidizing atmosphere. When the treatment is carried out at such a high temperature, fine diamond particles synthesized by a method, such as shock wave method, are oxidized, and therefore this process is not advantageous.
The inventors have variously investigated in order to solve the above described problems in the conventional methods and have found out that a combination use of ammonium nitrate and a specifically limited metal oxide can oxidize easily only graphite at a relatively low treating temperature in a short treating time without causing oxidation of diamond.